Neuromuscular activation strategies of voluntary andelectrically elicited muscle fatigue: Underlying mechanisms and clinicalimplications
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The clinical care of muscle fatigue with exercise therapies aim at quality of life improvement and usually involve unilateral exercises or neuromuscular electrical stimulation to compensate impaired muscle function in both healthy and health-compromised people. The overall objective of this thesis was to study the effect of neuromuscular activation strategies during muscle fatigue, endurance and muscle performance after unilateral fatiguing voluntary and electrically evoked contractions (NMES) in healthy and health-compromised populations such as children with cerebral palsy (CP) and patients with facioscapulohumeral muscular dystrophy (FSHD). A unilateral fatiguing exercise impaired the maximal torque production of the contralateral non-exercising limb, but the time course of torque production loss differed between limbs, being delayed in the non-exercising limb. This cross-over effect could be a compensatory strategy to reduce the gap of muscle performance between the weaker, exercised, limb and the non-exercised one likely to warrant interlimb coordination. A NMES protocol delivered over the nerve trunk, maintaining constant or increasing the current intensity induced similar peripheral and central adaptations of muscle fatigue. Therefore, for similar energy expenditure, maintaining the current intensity constant, as when increasing it, would yield the same adaptations to muscle fatigue. Also, we found that even though strength and time to task failure were similar in both groups, the children with CP exhibited a lower increase in EMG (electromyographic) amplitude compared to typically developing peers. This suggests that the children with CP were not able to recruit additional motor units to compensate for muscle fatigue during a sustained low-force level isometric elbow flexion contraction. Finally, no tibialis anterior muscle strength, endurance and motor function improvements were observed after a NMES training in a group of patients with FSHD. Maybe, the NMES protocol was not strenuous enough and/or parameters of stimulation were not adequate as no improvements were observed in healthy participants either. However, the group of patients with FSHD showed lower force losses during the 2-minute sustained MVC, suggesting that they were experiencing a lower amount of muscle fatigue compared to the healthy participants group. Limiting muscle fatigue and developing rehabilitation strategies is needed and further research should be conducted regarding the muscle activation patterns with muscle fatigue, especially in neuromuscular disorders. The potential beneficial cross-over and NMES effects should be further investigated with regards to the variety of neuromuscular disorders and the associated muscle activation impairments.